The hard magnetic disks used as memory media for storage of data in computers require an extremely high quality aluminum alloy substrate. The substrate depends on production of an especially high quality aluminum alloy sheet commonly referred to as disk stock. The magnetic disk substrate is blanked from this sheet, then processed through various thermal flattening, machining, lapping, polishing, chemical and anodizing operations before being coated with a thin film of magnetizable material. For example, such coatings may be applied by electroless or electrolytic plating or sputtering of cobalt-phosphorus or cobalt-nickel-phosphorus alloys directly on the aluminum alloy substrate, or by coating the substrate with iron oxide or other magnetic powder.
The magnetic transducer that reads and writes on such a disk "flies" within a micron or less of the rotating disk surface. An extremely high uniformity of surface is required to avoid crashes of such a flying head and to prevent dropouts of magnetic recorded data due to pinholes or the like in the recording film.
In recent years there has been an emphasis on producing disks with higher information density in order to increase their capacity. A higher density inherently necessitates a decrease in the area for each bit of magnetic information on the disk. The increased resolution requires decreasing thickness of the magnetizable film and reducing the distance from the flying head to the magnetizable film surface. These requirements can only be met on a surface which has minimal micro roughness and no asperities. Hence, a substrate material with excellent surface is a prerequisite.
The surface layers of the substrate must be mechanically, chemically and microstructurally homogeneous, thus assuring that after polishing and electrochemical treatments, the surface of the disk is extremely smooth and flat and has high magnetic uniformity. The surface layers should be free from defects, inclusions and segregation which may cause discontinuities in the surface topography or magnetic characteristics.
To make magnetic memories economically in commercial quantities, industrial scale melting and casting conditions must be used, and conventional aluminum plant rolling and heat treating equipment are important. The substrate must have suitable mechanical strength, corrosion resistance, modulus of elasticity, density, heat resistance and magnetic properties for reliable magnetic memory disks.
At present most disk stock is produced by classical methods involving casting of large direct chill ingots 300 to 600 millimeters thick and sufficiently wide to be rolled to sheet having a width of 1.1 meters. The cast ingot is hot rolled, followed by cold rolling and annealing operations to obtain the desired thickness and width.
An exemplary alloy for magnetic memory disk stock is 5086 having a magnesium content of about 4% and a manganese content of from 0.2 to 0.7%. These intentionally added alloying elements, along with some impurity elements typically present in the alloy, tend to form intermetallic compounds during the solidification process, the most prominent of these being various forms of Al-Fe-Mn and Mg-Si phases. Because of the relatively slow cooling rate with large ingots, the intermetallic compounds tend to be rather coarse with dimensions generally exceeding ten microns. These large intermetallic compound particles can be quite deleterious to the quality of a magnetic memory disk substrate. The intermetallic compounds are invariably harder than the aluminum alloy matrix and do not exhibit the same degree of plastic flow during rolling operations, hence they have a tendency to separate from the matrix, forming microscopic voids. The machining and lapping operations may leave the intermetallic particles as protuberances from the surface or may pull them out from the surface, leaving voids. Such surface particles or voids cause an electrochemical discontinuity which tends to disrupt the formation of a smooth, continuous anodic film during the electrochemical treatments. Discontinuities in the anodizing can be mimicked in the magnetic film applied to the substrate.
Grain refining materials can be added to the alloy used for casting of large ingots to produce a fine grain size. However, the intrinsically slow cooling rate produces a comparatively large dendrite arm spacing, allowing microsegregation to occur and producing microheterogeneity, particularly in the intermetallic compound distribution. This microsegregation is difficult to eliminate during subsequent processing and may result in uneven surface in the final disk substrate.
Another proposed technique for producing disk stock for magnetic media starts with continuous casting of aluminum alloy sheet. Techniques have been developed for continuously casting a variety of aluminum alloys into sheet less than 10 millimeters thick by introducing the metal through a pouring tip made of insulating material, into the nip of continuously rotating casting rolls which are water cooled, thereby freezing and somewhat hot rolling the cast sheet. This technique has proved rapid and economical for casting commercial purity aluminum sheet and a variety of aluminum alloys. However, continuous casting of aluminum alloys has not yet had an impact on the disk stock market.
The alloys of choice for making disk stock are 5086 and 5182 or the like. These alloys have proved particularly difficult to continuously cast with consistently high quality. No suitable technique has been developed for making production quantities of disk stock of these materials. Only narrow width, pilot plant scale quantities of metal have been produced. Even so the method has been dependent on tight control of alloy chemistry, which would be difficult to achieve in production conditions. Intermetallic segregation remains a problem since the largest particles are still of sufficient size to either protrude from the surface or leave voids, which in either case disrupt the formation of the anodic and magnetic films during electrochemical treatment.
Most significantly, prior continuous casting techniques for these alloys have not produced a completely homogeneous surface structure in the cast strip. Fluctuations during the casting process result in heterogeneity which results in a rippled appearance on the surface. Heterogeneity in the cast sheet may require a high temperature annealing treatment to ameliorate its effects.